1. Field of the Invention
The invention relates to an audio amplifier arrangement having a volume-dependent tone control and, incorporated in a signal path, a filtering device whose amplitude-frequency response has a low-pass slope in the frequency range between the bass and mid-range tones, and an amplifier stage incorporated in the signal path, while a feedback path extends from an output to an input of the amplifier stage.
2. Description of the Prior Art
Such an arrangement is known per se from Japanese Patent publication Kokai 61-158205. It may be incorporated, for example in a television or radio apparatus, a portable compact disc player or a "walkman" and may be denoted by the term "dynamic bass boost". It maintains the perceptual sound quality throughout the volume range by amplifying bass tones up to the order of 400 Hz and possibly treble tones from an order of 4 kHz at a low volume to a larger extent than mid-range tones which are between said frequency ranges.
FIG. 1 shows the known arrangement. A filtering device 2 is coupled to an amplifier 3 which in its rum is coupled to a loudspeaker 4 and to a detection device 5. The amplitude-frequency response of the filtering device 2 is switchable by means of a switching signal Sd between substantially two fixed characteristics. The output of the detection device 5 supplies the switching signal Sd which depends on the amplitude of the output signal of the amplifier 3 applied to the loudspeaker 4.
If the afore-mentioned amplitude exceeds a predetermined value, the switching signal Sd adjusts the filtering device 2 in such a way that it exhibits a substantially flat amplitude-frequency response. In this condition the known arrangement amplifies all tones to an equal extent. If the amplitude falls below the predetermined value, the switching signal Sd adjusts the filtering device 2 in such a way that it has an amplitude-frequency response at which mid-range tones are attenuated. In this condition the known arrangement amplifies both the bass tones and the treble tones to a relatively greater extent.
The known arrangement has a plurality of considerable shortcomings.
First, a relatively large number of components is required for realising the volume-dependent tone control. In practice, notably detection device 5 will often comprise a larger number of more expensive components than that shown in the embodiment in the afore-mentioned Patent publication. The relatively high cost price of the known arrangement renders it less suitable for use in low-cost consumer electronics.
Secondly, the tone control is adjusted in steps to the volume. The amplitude-frequency response of the known arrangement has only two characteristics. The deviation from an optimal characteristic associated with each volume may thus be relatively large. Moreover, a small variation of volume may result in abrupt transition from the one to the other amplitude-frequency response. This detracts from a high fidelity sound reproduction. Disturbing sound effects may occur, particularly when the volume fluctuates, instantaneously or not instantaneously, around this transition range.
Thirdly, a compromise must be found in the known arrangement between the rate at which the switching signal Sd responds to a change of volume on the one hand and the suppression of distorted audio signals in this switching signal on the other hand. If the afore-mentioned rate is too slow, the tone control will audibly trail the volume. If the afore-mentioned suppression is insufficient, the output signal of the known arrangement may be considerably distorted. Distorted audio signals in the switching signal may reach the output signal of the known arrangement either directly, or via mixing.
The compromise is inherent in the known facts: distorted audio signals are produced in the non-linear feedback path from the output to the input of the amplifier stage. By suppressing these audio signals, the reaction speed of the control signal decreases. Thus, the known arrangement cannot supply an output signal which is free from both additional distortion and disturbing sound effects.